Definition
The strength declaration
construct is used for modeling net type variables for a close correspondence
with physical wires.
Syntax
(Strength1, Strength0)
(Strength0, Strength1)
Strength1: (specifies the strength when
the net is driven with value “1”)
supply1, strong1, pull1, large1, weak1, medium1, small1, highz1
supply1, strong1, pull1, large1, weak1, medium1, small1, highz1
Strength0: (specifies the strength when
the net is driven with value “0”)
supply0, strong0, pull0, large0, weak0, medium0, small0, highz0
supply0, strong0, pull0, large0, weak0, medium0, small0, highz0
Description
Strengths can be used to resolve
which value should appear on a net or gate output. There are two types of
strengths:
drive strength (See Example 1) and charge
strength (See Example 2).
The drive strengths can be
used for nets (except trireg net), gates, and UDPs.
The charge strengths can be used only
for trireg nets.
The drive strength types
are supply, strong, pull, weak, and highz strengths.
The charge strength types
are large, medium and small strengths.
All strengths can be ordered by
their value. The supply strength is the strongest and
the highz strength is the weakest strength level. Strength value can
be displayed by system tasks ($display, $monitor - by using of the %v
characters).
Strength
|
Value
|
Value displayed by display
tasks
|
Supply
|
7
|
Su
|
Strong
|
6
|
St
|
Pull
|
5
|
Pu
|
Large
|
4
|
La
|
Weak
|
3
|
We
|
medium
|
2
|
Me
|
Small
|
1
|
Sm
|
Highz
|
0
|
HiZ
|
Table: Strengths ordered by decreasing value
There are different scenarios of
driving strength and is mentioned below –
If two or more drivers
drive a signal then it will have the value of the strongest driver (See Example
3). If two drivers of a net
have the same strength and value, then the net result will have the same value
and strength (See Example 4). If two drivers of a net
have the same strength but different values then signal value will be unknown
and it will have the same strength as both drivers (See Example 5). If one of the drivers of a
net has an H or L value, then signal value will be n1n2X,
where n1 is the strength value of the driver that has the smaller
strength, and n2 is strength value of driver that has the larger
strength (See Example 6).
** The combinations (highz0, highz1) and (highz1, highz0) are
illegal.
SAMPLE CODE1:
and (strong1, weak0) and1(out,
in1, in2);
Instance of and gate
with strong1 strength and weak0 strength specified.
SAMPLE CODE2:
trireg
(medium) t;
The charge strength declaration
for trireg net.
buf (strong1,
weak0) buff1 (y,
a);
buf (pull1, supply0) buff2 (y,
b);
If a = 0 and b = 0 then y will
be 0 with supply strength because both gates will set y to 0
and supply (7) strength has bigger value than weak (3)
strength.
If a = 0 and b = 1 then y will
be 1 with pull strength because buff1 will set y to 0
with weak (3) strength and buff2 will set y to 1
with pull (5) strength (pull strength is stronger than the weak
strength).
If a = 1 and b = 0 then y will
be 0 with supply strength because buff1 will set y to 1
with strong (6) strength and buff2 will set y to 0
with supply (7) strength (supply strength is stronger than the strong
strength).
If a = 1 and b = 1 then y will
be 1 with strong strength because buff1 will set y to 1
with strong (6) strength and buff2 will set y to 1
with pull (5) strength.
SAMPLE CODE4:
buf (strong1,
weak0)
buf2 (y, b);
If a = 1 and b = 1 then y will
be 1 with strong strength.
SAMPLE CODE5:
buf (strong1,
weak0)
buf1 (y, a);
buf (weak1, strong0)
buf2 (y, b);
If a = 1 and b = 0 then y will
be x with strong strength.
SAMPLE CODE6:
bufif0
(strong1,
weak0)
buf1 (y, i1, ctrl);
bufif0
(strong1,
weak0)
buf2 (y, i2, ctrl);
If ctrl = x, i1 = 0, and i2 = 1
then y will be x with 36X strength, because buf1 will set y to L
with strong strength (StL - 6) and buf2 will set y to H
with weak strength (WeH - 3).
Important Notes
If one of the drivers has an H or L value, then the output value will be
X.
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